1. Field of the Invention
This invention generally relates to a high-speed optical transmitter for a fiber optic system using a coupled multi-cavity filter as a discriminator to convert frequency modulated signal to a substantially amplitude modulated signal.
2. General Background
Fiber optic communication systems use a variety of transmitters to convert electrical digital bits of information into optical signals that are sent through optical fibers. On the other end of the optical fiber is a receiver that converts the optical signal to an electrical signal. The transmitters modulate the signals to form bits of is and Os so that information or data may be carried through the optical fiber. There are a variety of transmitters that modulate the signal in different ways. For example, there are directly modulated transmitters and indirectly modulated transmitters. The directly modulated transmitters offer a compact system having large response to modulation and are integrateable; The directly modulated transmitters are also generally less expensive than the externally modulated transmitters, which require an intensity modulator, usually LiNbO3, following the laser.
One of the drawbacks of a directly modulated transmitter, however, is that its output is highly chirped. Chirp is the rapid change in optical frequency or phase that accompanies intensity modulated signal. Chirped pulses become distorted after propagation through tens of km of dispersive optical fiber, increasing system power penalties to unacceptable levels. This has limited the use of directly modulated laser transmitters to applications with limited distances of tens of km at 2.5 Gb/s as described by P. J. Corvini and T. L. Koch, Journal of Lightwave Technology vol. LT-5, no. 11, 1591 (1987). For higher bit rate applications, the use of directly modulated transmitters may be limited to even shorter distances.
An alternative to directly modulating the laser source is using a laser source that produces a partially frequency modulated signal and an optical discriminator as discussed in UK Patent GB2107147A by R. E. Epworth. In this technique, the laser is initially biased to a current level high above threshold. A partial amplitude modulation of the bias current is applied so that the average power output remains high. The partial amplitude modulation also leads to a partial but significant modulation in the frequency of the laser output, synchronous with the power amplitude changes. This partially frequency modulated output may then be applied to a filter, such as a Fabry Perot filter, which is tuned to allow light only at certain frequencies to pass through. This way, a partially frequency modulated signal is converted into a substantially amplitude modulated signal. That is, frequency modulation is converted into amplitude modulation. This conversion increases the extinction ratio of the input signal and further reduces the chirp.
Since Epworth, a number of variations from his technique have been applied to increase the extinction ratio from the signal output of the laser. For example, N. Henmi describes a very similar system in U.S. Pat. No. 4,805,235, also using a free-space interferometer. Huber U.S. Pat. No. 5,416,629, Mahgerefteh U.S. Pat. Ser. No. 6,104,851, and Brenner U.S. Pat. No. 6,115,403 use a fiber Bragg grating discriminator in similar configurations. In the more recent work, it has also been recognized that a frequency-modulated transmitter with a frequency discriminator produces an output with lower chirp, which reduces the pulse distortion upon propagation through a communication fiber. Chirp is a time dependent frequency variation of an optical signal and generally increases the optical bandwidth of a signal beyond the Fourier-transform limit. Chirp can either improve or degrade the optical pulse shape after propagation through a dispersive fiber, depending on the sign and exact nature of the chirp. In the conventional directly modulated laser transmitter, chirp causes severe pulse distortion upon propagation through the optical fiber. This is because the speed of light in the dispersive medium is frequency dependent, frequency variations of pulses may undergo different time delays, and thus the pulse may be distorted. If the propagation distance through the medium is long as in the case of optical fibers, the pulse may be dispersed in time and its width broadened, which has an undesirable effect.
In these systems, the discriminator is operated to increase the extinction ratio of the input signal or to remove some component of the signal in favor of the other. As such, only the amplitude variation of the discriminator has been utilized. In addition, these systems have mainly dealt with lower bit rate applications. At low bit rates, the spectrum of a modulated laser biased above its threshold includes two carriers, each carrying the digital signal used to modulate the laser. The wavelengths of the two peaks are separated by 10 GHz to 20 GHz depending on the laser and the bias. Hence, a variety of optical discriminators, Fabry-Perot, Mach-Zehnder, etc. may be used to resolve the two peaks, generally discarding the 0s bits and keeping the 1s bits, thereby increasing the extinction ratio at the output.
A Fabry-Perot filter is formed by two partially reflecting mirror surfaces, which are separated by a small gap on the order of a few micrometers. The cavity is either an air gap or a solid material formed by deposition or cut and polish method. The transmission of a Fabry-Perot filter consists of periodic peaks in optical frequency separated by the so-called free-spectral range (FSR), which is inversely proportional to the thickness of the gap. The steepness of the peaks is determined by the reflectivity of the two mirrors. However, the steeper the transmission edges, the narrower the pass-band of the filter. As such, Fabry-Perot filter may provide the steeper transmission edges or slope, but it does not provide the broad enough bandwidth for high bit rate applications such as 10 Gb/s.
At higher bit rates, the spectrum of the frequency modulated signal becomes more complicated and the choice of discriminators that may be used is limited. At high bit rates around 10 Gb/s, the information bandwidth becomes comparable to the frequency excursion of the laser, which is typically around 10 GHz. In addition, the transient chirp that arises at the transitions between 1s and 0s broadens to complicate the spectrum further. In order to separate the 1 and 0 bits with the extinction ratio of 10 dB, the slope of the discriminator should be greater than 1 dB/GHz, while passing 10 Gb/s information. Under this performance criteria, a Fabry-Perot filter may not work because the bandwidth and slope characteristics of Fabry-Perot filters are such that the steeper the transmission edges, the narrower the pass-bandwidth of the filter. As illustrated in FIGS. 1A and 1B, a Fabry-Perot discriminator with 1 dB/GHz slope may only have about 3 GHz bandwidth. Such limited bandwidth can severely distort a 10 Gb/s signal such that the FM modulated transmitter with a Fabry-Perot filter may not work at this bit rate. Others have tried Fiber Bragg gratings for high bit rate applications, but fiber gratings are sensitive to temperature and require a separate package with temperature stabilization. Therefore, there still is a need for a compact frequency discriminator that can operate with a FM modulated source at high bit rates such as 10 Gb/s.